The development of dental caries in children provides a disease model that is well suited to a metagenomic investigation since the timeframe of caries development can be monitored within a defined study period during which longitudinal studies can be conducted. Dental caries has a well established link to discrete, but only partially characterized changes in the dental plaque microbiome. Our study endeavors to define the relative abundance of many species comprising the dental plaque microbiome in 60- 70 twin pairs discordant for dental caries in association and longitudinal studies. The study power afforded by the use of discordant twin pairs for dental caries is substantially greater than similar studies using unrelated individuals. Next, we will compare the caries-free and caries-active microbiomes by deep metagenomic sequencing to identify differentially abundant genes and metabolic pathways. Lastly, we will determine the functional relevance of differentially abundant species, strains and genes by conducting targeted meta-transcriptomic studies. The statistical significance of all of the datasets will be determined which will allow for the widespread identification of species, strains, genes and transcripts associated with dental caries and dental health. These approaches are likely to enhance our understanding of dental caries onset and development and may enable in the future the development of novel treatment strategies. PUBLIC HEALTH RELEVANCE: Our project directly addresses the identification of microbes that are responsible for the onset and development of dental caries. The data generated will allow us to identify numerous diagnostic markers for dental health and disease. This work may also define novel therapies for dental caries.